1. Field of the Invention
The invention pertains to the field of variable camshaft timing (VCT) systems. More particularly, the invention pertains to a control method which prevents integrator wind-up when operating VCT at or near its physical stops.
2. Description of Related Art
It is known in the art to use a negative feedback loop for controlling variable camshaft timing (VCT) systems. U.S. Pat. No. 5,289,805 describes an improved closed loop feedback system for a VCT system. The same patent further teaches a robust control law used in the closed loop feedback system for a VCT system. The control law includes a phase integration (PI) block and a phase lead block. FIGS. 1 and 2 show the feedback loop and the control law respectively.
Referring to FIG. 1, a prior art feedback loop 10 is shown. The control objective of feedback loop 10 is to have the VCT phaser at the correct phase (set point 12) and the phase rate of change is zero. In this state, the spool valve 14 is in its null position and no fluid flows between two fluid holding chambers of a phaser (not shown). A computer program product which utilizes the dynamic state of the VCT mechanism is used to accomplish the above state.
The VCT closed-loop control mechanism is achieved by measuring a camshaft phase shift .xcex80 16, and comparing the same to the desired set point r 12. The VCT mechanism is in turn adjusted so that the phaser achieves a position which is determined by the set point r 12. A control law 18 compares the set point 12 to the phase shift xcex80 16. The compared result is used as a reference to issue commands to a solenoid 20 to position the spool 14. This positioning of spool 14 occurs when the phase error (the difference between set point r 12 and phase shift 20) is non-zero.
The spool 14 is moved toward a first direction (e.g. right) if the phase error is positive (retard) and to a second direction (e.g. left) if the phase error is negative (advance). When the phase error is zero, the VCT phase equals the set point r 12 so the spool 14 is held in the null position such that no fluid flows within the spool valve.
Camshaft and crankshaft measurement pulses in the VCT system are generated by camshaft and crankshaft pulse wheels 22 and 24, respectively. As the crankshaft (not shown) and camshaft (also not shown) rotate, wheels 22, 24 rotate along with them. The wheels 22, 24 possess teeth which can be sensed and measured by sensors according to measurement pulses generated by the sensors. The measurement pulses are detected by camshaft and crankshaft measurement pulse sensors 22a and 24a, respectively. The sensed pulses are used by a phase measurement device 26. A measurement phase difference is then determined. The phase difference is defined as the time from successive crank-to-cam pulses, divided by the time for an entire revolution and multiplied by 360xc2x0. In other words, the angular position difference is referenced to the difference between the cam shaft and the crank shaft with and without the Variable Cam Timing system. The measured phase difference may be expressed as xcex80 16. This phase difference is then supplied to the control law 18 for reaching the desired spool position.
A control law 18 of the closed-loop 10 is described in U.S. Pat. No. 5,184,578 and is hereby incorporate herein by reference. A simplified depiction of the control law is shown in FIG. 2. Measured phase 26 is subjected to the control law 18 initially at block 30 wherein proportional-integral (PI) process occurs. Typically PI process is subdivided into two sub-processes. The first sub-process includes an amplification action; and the second sub-process includes an integration action. Measured VCT phase is further subjected to phase compensation at block 32. One of the drawbacks of the above prior art approach is that at or near the physical stops of the phaser, the prior art method cannot accurately indicate the exact physical position of the phaser. One of the undesirable side effects is that the Integrator of the control law would wind-up. This phenomenon often occurs when an internal combustion engine system goes under prolonged use in which physical components may change their characteristics. For example, the timing chain may be stretched.
Therefore, it is desirable to provide a control method and system, which prevents integrator wind-up when operating VCT phaser at or near its physical stops.
A method involving a VCT phaser to automatically learn VCT phaser physical stops while the engine is running is provided.
A method involving a VCT phaser for eliminating integrator winding up is provided.
A method involving a VCT phaser for learning a value of a PI controller output (E1) at steady state is provided.
A method involving a VCT phaser for learning a value of a PI controller output (E1) at physical stops is provided.
A method involving a VCT phaser which determines when to reset an integrator if the phaser approaches its physical stops is provided.
A method involving a VCT phaser which determines when to reset a compensator if the phaser approaches its physical stops is provided.
A method involving a VCT phaser to self correct a mistakenly learned physical stop is provided.
A computer program product involving a VCT phaser, which can automatically learn VCT physical stops while engine is running, is provided.
A computer program product involving a VCT phaser for eliminating integrator winding up is provided.
A computer program product involving a VCT phaser for learning a value of a PI controller output (E1) at steady state is provided.
A computer program product involving a VCT phaser for learning a value of a PI controller output (E1) at physical stops is provided.
A computer program product involving a VCT phaser which determines when to reset an integrator if the phaser approaches its physical stops is provided.
A computer program product involving a VCT phaser which determines when to reset a compensator if the phaser approaches its physical stops is provided.
A computer program product involving a VCT phaser to self correct a mistakenly learned physical stop is provided.
Accordingly, a method for a VCT feed back control system is provided. The method includes the steps of: a) providing a set of tooth pulses; b) filtering said set of tooth pulses; c) identifying a phaser that is not moving; d) determining whether the non-moving phaser is at stop state or steady state; and e) learning the phaser physical stop.
Accordingly a VCT feed back control system is provided. The system includes: a) a variable force solenoid; b) a spool valve capable of being engaged by said solenoid; c) a VCT phaser disposed to determine a set of positions, wherein a set of relationships between a crank shaft and cam shaft is determined, said VCT phaser being controllable by positions of said spool; and d) a controller. The controller includes a control law disposed to receive a set point and capable of controlling said variable force solenoid; a filter for filtering position signals of a rotating shaft; and an identifier for receiving the filtered position signal, identifying said VCT phaser state, and generate a reset signal to reset said control law.